Process for making high initial permeability iron-nickel alloys

ABSTRACT

A PROCESS IS DESCRIBED IN WHICH A NICKEL-IRON MAGNETIC ALLOY IS HEAT TREATED AT RELATIVELY LOW ANNIALING TEMPERATURES TO PRODUCE A PRODUCT WITH A HIGH INITIAL PERMEABILITY. CRITICAL STEPS INCLUDE CONTROL OF THE SULFUR CONTENT AND A HEAT TREATMENT WHICH PRODUCES, IN THE MATERIAL, A COARSE GRAINED SECONDARY RECRYSTALLIZED MICROSTRUCTURE.

States US. Cl. 148-120 7 Claims ABSTRACT OF THE DISCLOSURE A process is described in which a nickel-iron magnetic alloy is heat treated at relatively low annealing temperatures to produce a product with a high initial permeability. Critical steps include control of the sulfur content and a heat treatment which produces, in the material, a coarse grained secondary recrystallized microstructure.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to nickel-iron alloys of the fifty-fifty composition, a commercial variety of which is known as Hipernik and more specifically to an improved method for obtaining high initial permeability in an iron-50% nickel alloy composition.

Description of the prior art During the last fifty years a considerable amount of research has been conducted on iron-nickel alloys wherein the nickel content ranged from minimal amounts to values as much as 80% nickel. As a result it is well known that in the high permeability alloys very poor magnetic properties are exhibited in the as-fabricated condition and their best magnetic properties can only be developed by heat treatment. Yensen in US. Pat. 1,807,021 described a heat treatment wherein the maximum permeability exhibited by a composition containing about equal parts of iron and nickel was improved by a factor of about four. This heat treatment included a high temperature annealing at 1150 to 1300" C. in a hydrogen atmosphere with a controlled cooling from the annealing temperature. This research made it clear that annealing improves not only the maximum permeability but other properties such as coercive force. Moreover, the initial permeability has also been improved by recrystallizing the microstructure and by relieving the stresses introduced to the materials by cold working. In addition thereto, it has also been found that common impurities such as carbon, oxygen and sulfur must not be present to a substantial extent or magnetic properties will be poor. Consequently, any heat treatment which has been involved to improve the properties was conducted in a protective atmosphere which would not add injurious elements to the material. As a result thereof it has been found that in commercial usage the preferred atmosphere is dry hydrogen with a dew point of as low as about 25 C. Such heat treatment apparently purifies the alloy as well as provides the same with a clean bright surface thereon. As a result of the various and sundry refinements, present day commercially available compositions having a thickness of about 0.014 inch will exhibit an initial permeability when measured at 40 gausses of about 10,500. Further, commercal present day requirements indicate the need for higher initial permeability to be exhibited by these compositions in order to meet desired design requirements.

SUMMARY OF THE INVENTION The present invention relates to a method for producing high initial permeability in an alloy having be tween about 46% and about 52% by weight of nickel and the balance essentially iron with incidental impurities. The method contemplated includes making a melt of the desired composition in which the as-cast sulfur content is controlled to an amount of less than 10 parts per million. Thereafter, the as-cast material is fabricated as by hot and cold rolling to the desired thickness, usually not in excess of 0.014 inch in thickness, and its thereafter subjected to a heat treatment to produce a coarse secondary recrystallized microstructure. The heat treatment preferably takes place at a temperature Within the range between about 1000 C. and about 1100 C. in a protective non-oxidizing and non-carburizing atmosphere. By thus controlling the final sulfur content to a value of less than 10 parts per million and by heat treating the material at a temperature Within the range between 1000 C. and 1100 C. to produce a coarse grain secondary recrystallized microstructure, the material exhibits a high initial permeability which in most cases exceeds 10,500 when measured at 40 gausses and a frequency of 60 hertz.

Since the sulfur content is removed prior to the final annealing it becomes apparent that an additional advantage is obtained by permitting final anneals to take place at much lower temperatures than required previously. This means that the user of the nickel-iron alloy has less of a capital investment in annealing facilities and lower maintenance cost on furnaces because of the lower annealing temperatures. Moreover, it has also been found that a more uniform quality to the magnetic characteristics is obtained because sulfur is difficult to remove uniformly in commercial anneals where laminations are stacked against one another and there is a channeling of flow of the annealing atmosphere.

It is an object of the present invention to produce high initial permeability in an alloy having a composition of between about 46% and about 52% nickel and the balance essentially iron and incidental impurities.

Another object of the present invention is to provide a method for obtaining high initial permeability by controlling the sulfur content of the as-cast material thereby enabling heat treatment at lower temperatures to obtain equivalent magnetic characteristics.

A more specific object of the present invention is to provide a method of producing high initial permeability in alloys containing between about 46% and about 52% by weight of nickel with the balance essentially iron and incidental impurities by controlling the sulfur content to a predetermined low level and thereafter subjecting the material to a heat treatment which produces a secondary recrystallized microstructure exhibiting a very coarse grain size.

Other objects of this invention will become apparent to those skilled in the art when read in conjunction with the following description.

DESCRIPTION OF THE PREFERRED EMBODIMENT The material to which the process of the present invention is applicable is the standard commercial ironnickel alloy composition and which includes between about 46% and about 52% by weight of nickel, 1.0% maximum manganese, 0.6% maximum silicon, less than about 0.08% carbon, less than about 25 parts per million oxygen, less than about 0.010% sulfur and the balance essentially iron with incidental impurities.

As has been noted hereinbefore, it is preferred to control the sulfur content of the material to a level of less than about 10 parts per million in the as-cast ingot. Such low sulfur contents are necessary in order to develop the high initial permeability in these materials. However, the sulfur content can be removed during processing where it is found impractical to produce an as-cast material having less than about parts per million sulfur. In such an instance it is preferred to maintain the as-cast sulfur content of not greater than 0.010%. Thereafter the material after hot working can be subjected to a desulfurization heat treatment wherein the sulfur content is removed to a level of less than about 10 parts per million in the finished product. The actual method employed for controlling the sulfur in the finished product to less than about 10 parts per million sulfur, that is, by melting practices wherein such sulfur levels are obtained or in the application of a desulfurization heat treatment follow ing hot working, will be governed by the particular economics involved.

Material having the above described composition may be made in any well known manner. Air induction melting and electric furnace melting have proved to be successful so long as the as-cast material will have a chemical composition within the limits described hereinbefore. The molten metal can be teemed to form an electrode which may be thereafter vacuum consumable electric arc remelted in manners well known in the art. While such remelting is not necessary, nonetheless it is preferred in practicing the method of the present invention.

Following the melting of the ingredients to the desired composition the ingots may be forged or hot rolled to a desired intermediate gauge thickness. Thereafter, the materials are preferably descaled and conditioned for final cold reduction. While the cold reduction may take place in one or more passes it is preferred to have a final cold reduction without any intermediate reheat treatment to reduce the material to finish gauge, that is, a thickness of less than about 0.014 inch in thickness. Such cold reduction should be accomplished so that the total reduction in cross-sectional area from hot work gauge to final finish gauge should be at least 90%. Giving the material such a cold reduction of at least 90% to finish gauge is believed to be elfective for obtaining a very coarse grain size in the secondary recrystallized microstructure as will be described more fully hereinafter.

As an alternate to the foregoing processing it is possible to melt the desired composition having a sulfur content in excess of 10 parts per million but less than about 0.010% sulfur. This material, as preferably vacuum consumable are remelted may be hot rolled to a thickness of, for example, 0.150 inch in thickness. As the material has been hot worked it is thereafter conditioned by pickling or mechanically removing the scale from the surface and thereafter subjected to a desulfurization heat treatment, such desulfurization heat treatment being accomplished by heating the material to a temperature within the range between about 1100 C. and 1300 C. in a hydrogen atmosphere having a dew point of less than about 25 C. Thereafter the material can be subjected to a final cold rolling once again to effect a reduction in the cross-sectional area of at least 90% from hot rolled gauge to finish gauge thickness.

The material of finish gauge thickness can be punched to the desired lamination shape following which it is preferred to deburr the individual laminations, apply a slurry of MgO in a water vehicle to the surface thereof, dry the coating and thereafter subject the laminations to a secondary recrystallization heat treatment. In particular it should be pointed out that it is preferred to heat treat the material at a emperature within the range between 1000 C. and 1100 C. for a time period of at least one hour and preferably for time periods of between about 4 hours and about sixteen hours. During such heat treatment the material is subjected to a protective atmosphere preferably in a form of hydrogen having a dew point of less than about -25 C. During such heat treatment the materials processed in accordance with the various steps outlined hereinbefore will exhibit a microstructure characterized by a secondary recrystallization in which the grains have assumed a very coarse texture and have a size in excess of the finish gauge thickness. Thus the time at temperature is carried out in such a manner as to produce the coarse grain size of the secondary recrystallized microstructure, it having been found that the larger the grain size of the secondary recrystallized microstructure, the higher the initial permeability exhibited by these materials treated in accordance with the method of the present invention.

In order to more clearly demonstrate the advantages of the method of the present invention reference may be had to Table I set forth hereinafter which lists the ingot analysis of two heats of commercial iron-nickel alloy each totaling 5500 pounds, which were melted in an air induction furnace having a basic lining. The first listed heat HKV 8054 represents a standard carbon deoxidation melting practice whereas the second heat HKV 3100' represents a modified melting practice which yielded a low sulfur and oxygen content.

TABLE I [Chemical analysis (percent by wt.)]

HKV 8054 KHV 3100 In both cases the molten metal was teemed to form an electrode for vacuum consumable arc remelting. Thereafter each ingot was subjected to a vacuum consumable arc remelting following which the final ingots were forged to a slab which was thereafter hot rolled to a thickness in the range between 0.120 and 0.250 inch in thickness. The hot rolled bands were thereafter conditioned, in this instance by picking, to remove the mill scale therefrom following which each band was given a cold reduction of at least to a finish thickness of 0.014 inch. The finished gauge material was punched in a standard punch press and standard test punchings, known as DUs, were produced for the determination of the initial permeability. These DUs were deburred and thereafter coated with a water slurry containing MgO, dried and thereafter subjected to an annealing heat treatment in a hydrogen atmosphere prior to magnetic testing. During such annealing at the various temperatures, the hydrogen dew point was maintained at less than 50 C. All of the materials were subjected to the annealing heat treatment for a time period of four hours and the hydrogen was maintained at the given dew point level at least during the entire period at which the DUs were maintained at a temperature in excess of 800 C.

Reference is directed to Table II which illustrates the annealing temperature, chemical analysis, microstructure and the initial permeability of the materials for each of the two heats set forth-hereinbefore in Table I.

TAB LE II Sulfur Initial analysis permeability, (p.p.m.) u at 40 B Microstrueture Annealing HKV HKV HKV HKV temp. C.) 8054 3100 HKV 8054 HKV 3100 8054 3100 900 28 8 Primary re ystallization Primary recrystallization 890 2,100 32 5. 5 Coarse secondary recrystallization Coarse secondary recrystallization--. 3, 550 9, 390 17 7,160 11, 360 1.1 14,890 11. 870

From the test results recorded in Table II it is apparent that for each heat an increase in the annealing temperature from 900 C. to 1000 C. did not markedly change the sulfur level. However, the increase in the permeability is associated with the change from a fine 6 Reference is now directed to Table IV which sets forth the sulfur analysis, the microstructure and the initial permeability exhibited by the materials after annealing for four hours in hydrogen at the annealing temperature indicated.

TABLE IV Initial Sulfur analysis permeability,

(p.p.m.) Mierostructure p at 40 B Std. Desulf. Std. Desulf. Std. Desulf.

Annealin tem 0.:

900 P 50 2. Primary recrystallization Primary recrystallization 770 4, 400 32 3. 0 Coarse secondary recrystallization. Fine secondary recrystallization. 2, 950 7, 900 24 2.2 -.do- 6,130 10,600 6.3 0.5 do-. 13,170 11, 775

grain primary recrystallized microstructure to a coarse grain secondary recrystallized microstructure. Thus, for a given anneal at 900 C. or at 1000 C. heat HKV 3100 had a higher permeability because of its lower sulfur content. It is noted however that the permeability for L heat HKV 8054 increased at a higher rate with an increase in the annealing temperature to a higher value. This may be explained on the basis that the large incremental change in both microstructure and final sulfur content has occurred to this heat as compared with heat HKV 3100 to a final sulfur content of a much lower value. The experience has been that given the same final sulfur content in heat HKV 3100 it would be expected that equivalent results would be obtained. From the foregoing it becomes clear that lower temperatures can be employed where the sulfur content is controlled in order to get high initial permeability in the material falling within the scope of the chemical analysis described hereinbefore. Thus, it becomes clear that it is necessary to not only control the sulfur content to values of less than about parts per million, but in addition thereto, it is also necessary to obtain a microstructure characterized by coarse secondary recrystallized grains following the cooling from the final heat treatment.

As stated hereinbefore it may be desirable to control the melting of the ingredients to a much higher sulfur level and thereafter subject the material to a desulfurizing anneal during the reduction of the material from ingot to final gauge product. In order to demostrate the feasibility and process of the present invention reference may be had to Table III which details the chemical analysis of a standard heat of the iron-nickel composition which was melted in an air induction furnace using standard carbon deoxidation practice.

TABLE III HKV 3332 HKV 3332 hot band ingot desulfurizatiou The ingot produced from the air induction melting weighed 5500 pounds and was teemed in the form of an electrode for vacuum consumable arc remelting. The remelted ingot, having the foregoing composition was thereafter forged and hot rolled to a thickness of 0.150 inch in thickness. A section of the hot rolled strip was pickled and subjected to a desulfurization heat treatment by annealing at a temperature of 1200" C. in hydrogen having a dew point of less than 50 C. Both standard and desulfurized material were thereafter cold rolled to 0.014 inch in thickness from which standard DUs were punched for measurement of the initial permeability. Each of the sets of punched laminations were deburred, coated with a water slurry of MgO, dried and stacked in lamination form following which they were annealed at various temperatures in hydrogen having a dew point of less than -50 C. prior to magnetic testing.

From an examination of the data set forth in Table IV it is noted that annealing these materials at a temperature of 900 C. was ineffective for producing a secondary recrystallized microstructure. As a result thereof the standard material exhibited a permeability of 770 whereas the previously desulfurized material exhibited a permeability of 4400. However, if the sulfur levels are compared it is noted that the desulfurized material having about two parts per million of sulfur, nonetheless, displayed a much higher initial permeability than that of the standard material, having about 50 parts per million sulfur contained therein. On the other hand where the annealing temperature was increased to 1000 C. it is noted that an improvement has occurred to the standard processed material even though the sulfur level is at 32 parts per million. This improvement in the magnetic characteristics is attributable to the fact that the microstructure is characterized by having a very coarse secondary recrystallized microstructure. While an improvement is also noted insofar as the desulfurized material is concerned, nonetheless the fine grain size is responsible for not showing a better improvement in properties than illustrated by the material annealed at 900 C. The standard processed material after 1200 C. anneal has an even higher permeability than the desulfurized material; the principal reason for this reversal is that the sulfur levels approach each other and the grain size of the secondaries in the standard material were much coarser than the fine grain size of the secondary recrystallized microstructure of the desulfurized material. Consequently, a much coarser secondary recrystallized material for the desulfurized material, would exhibit a significant increase in the permeability after annealing at 1000 C. or 1100 C.

From the foregoing it is apparent that the process of the present invention is effective for producing high initial permeability in iron-nickel alloys with increased uniformity by heat treatment at lower temperatures than presently practiced. Such advantages are obtained only when the sulfur content is maintained at low levels, that is, below about 10 parts per million in the finished product and the material has undergone complete secondary recrystallization in which a large grain size is obtained therein. The grain size is preferred to be larger than the finish thickness of the material. It is apparent that so long as the sulfur content is maintained at the given low levels, either by melting the same to the desired low levels or by subjecting the material after hot rolling to an intermediate treatment wherein the sulfur content is removed to a value of less than 10 parts per million, the process of the present invention is effective for producing outstandingly high initial permeability to this class of materials. Substantially the same results can be obtained by powder metallurgical techniques using either sulfur free pure powders or accomplishing the sulfur purification during high temperature sintering process. In either event it still becomes critical that the sulfur content be maintained at the required low level, that is, less than 10 parts per million and in addition thereto, the material exhibits a secondary recrystallized microstructure which is characterized by a coarse grain size.

We claim as our invention:

1. In the method of producing by a relatively low temperature anneal, a high initial permeability in an alloy having between about 46% and about 52% nickel and the balance iron with incidental impurities, the steps comprising, making a melt of the desired composition in which the as-cast sulfur content is controlled to an amount of less than p.p.m., fabricating the metal to the desired thickness of not in excess of about 0.014 inch in thickness and thereafter subjecting the material to a secondary recrystallization heat treatment at a temperature Within the range between about 1000 C. and 1100 C. in a protective atmosphere.

2. The method of claim 1 in which the protectivee atmosphere is hydrogen having an exit dew point of less than -25 C.

3. In the method of producing high initial permeabilities to iron-nickel alloys, the steps comprising, making a melt of material having a composition containing between about 46% and about 52% nickel, up to about 1% manganese, up to 0.60% silicon, not more than about 0.08% carbon, less than 25 p.p.m. oxygen, less than about 0.10% sulfur and the balance essentially iron with residual impurities, casting said melt, hot working the casting to a thickness between about 0.080 and about 0.250 inch in thickness, subjecting the hot Worked material to a desulfurizing anneal at a temperature within the range between 1100 C. and 1300 C. in a protective atmos phere, cold working the material to finish gauge and thereafter subjecting the material to a secondary recrystallization anneal at a temperature within the range between 1000 C. and 1100 C. in a protective atmosphere.

4. The method of claim 3 in which the desulfurizing anneal takes place in an atmosphere of hydrogen having a dew point of less than 25 C.

5. The method of claim 3 in which the secondary recrystallization anneal takes place in hydrogen having a dew point of less than about 25 C. and for a time period of between about one and 64 hours and preferably between about 4 hours and about 16 hours with the shorter time periods being associated with the higher temperature and the longer time periods being associated with the lower temperature of the secondary recrystallization annealing temperature range.

6. The method of claim 3 in which the cold working is accomplished in one or more steps to eifect a total reduction in cross-sectional area of of the hot worked cross sectional area.

7. In the method of producing high initial permeabil ity in an alloy having between about 46% and about 52% nickel and the balance iron with incidental impurities, the steps comprising, making a melt of the desired composition in which the as-cast sulfur content is controlled to an amount of less than 10 p.p.m., hot working theascast material to a thickness within the range between 0.120 and 0.250 inch in thickness, descaling the hot rolled band, cold working to finish gauge to effect a reduction in cross-sectional area of at least 90% of hot worked thickness and subjecting the finish gauge material to a secondary recrystallization heat treatment at a temperature within the range between 1000 C. and 1100 C. in a protective atmosphere.

References Cited UNITED STATES PATENTS 1,807,021 5/1931 Yenson 148-122X 1,862,357 6/1932 Ruder 148l20X 2,558,104 6/1951 Scharschu 1482X 2,569,468 10/1951 Gaugler 148l22X 3,247,031 4/1966 Littmann et al. 148--120 3,297,434 1/1967 Littmann et a1. 148l21X L. DEWAYNE, RUTLEDGE, Primary Examiner- G. K. WHITE, Assistant Examiner US. Cl. X.R. 

